According to data from the United Nations Food and Agriculture Organization (FAO), world seafood consumption has doubled every 20 to 25 years since 1950 [FAO. 2007 Fishery and Aquaculture Statistics. Rome: Food and Agriculture Organization of the United Nations, 2007]. Today fish from most traditional fisheries are being harvested at maximum yields, while fish populations in almost all commercial marine fisheries around the world arc undergoing a dramatic decrease due to over-fishing. In the United States seafood consumption has increased 60% since 1960 and approximately 85% of this demand each year is met by imported fish and shellfish resulting in approximately $7 billion annual trade deficits in edible fishery products [NOAA. Fish Watch U.S. Seafood Fact. In; 2010. See world wide web address: nmfs.noaa.gov/fishwatch/trade_and_aquaculture.htm].
Aquaculture, also known as “fish farming,” is becoming increasingly important to offset this deficiency in aquatic foods production, and dramatic increases in aquaculture production have been made over the last few decades. From 2002 to 2007 annual aquaculture production increased from 36.8 million tons to 50.3 million tons with a total value of $87 billion [FAO. 2007 Fishery and Aquaculture Statistics. Rome: Food and Agriculture Organization of the United Nations, 2007]. As this shift in dependence from wild populations to artificially propagated aquatic species continues, optimization of aquaculture methods will be necessary to maximize food production.
According to the Food and Agriculture Organization (FAO) of the United Nations, nearly 70% of the fish species in the world's commercial marine fisheries are now fully exploited, overexploited or depleted. Based on anticipated population growth, it is estimated that the world's demand for seafood will double by the year 2025. Therefore, a growing gap is developing between demand and supply of fisheries products, which results in a growing seafood deficit. Even the most favorable estimates project that in the year 2025, the global demand for seafood will be twice as much as the commercial fisheries will harvest.
The same trend exists in the United States marketplace. Per capita seafood consumption is on the rise, but United States seafood harvests are not increasing to meet the demand. Moreover, only 10% of the seafood consumed in the United States comes from domestic aquaculture and the United States ranks only tenth in the world in the value of its aquaculture production. As a result, the United States is overwhelmingly dependent on imported seafood, such as gilthead and seabream, with more than half of its supplies coming from overseas.
Worldwide, it is estimated that in order to close the increasing gap between the demand and supply of commercially produced fish, aquaculture will need to augment production five-fold during the next two and half decades. While there is a need to increase aquaculture production globally and in the United States, it is clear that fish farming must develop as a sustainable industry without having an adverse impact on the environment.
In commercial fish species where sexual maturation occurs before the fish has reached market size, energy is spent on gonadal growth instead of muscle growth. Sterility increases the conversion of food energy to muscle (thereby resulting in larger fish fillets) and minimizes food energy diverted for development of the gonads. Another advantage of fish sterilization is that it minimizes the potential negative impact of genetically modified fish on the environment, because without sterilization escape of genetically-modified cultured fish may threaten the ecological balance or lead to genetic contamination of wild populations. This threat will become even greater as transgenic fish are raised in commercial operations in the United States and abroad.
Aquaculture experts around the world agree that a mechanically simple, but effective, process that bypasses the traditional modes of inducing sterility would increase production efficiency, profitability and biosecurity in commercial aquaculture. Sterilizing transgenic or genetically-selected fish will minimize the possibility these fish propagating in the wild, an especially important consideration in light of alarming reports of interbreeding between escaped animals and wild populations of the same species, which seem to be increasing in areas of intensive farming (Volpe et al., 2001; Carr et al., 1997).
Several methods currently exist to induce sterility in fish. One method is manipulation of the chromosome number by triploidization or interspecies hybridization and another is the disruption of the gonadotropin-releasing hormone (GnRH) pathway using antisense RNA or treatment with gamma amino butyric acid (GABA).
Chromosome set manipulation for the production of triploid sterile populations is used but it is a cumbersome procedure that must be individually developed for each species. Furthermore, generation of triploids does not always result in sterility. In induced-triploid rainbow trout (Arai, 2001) and Atlantic salmon (Donaldson and Benfey, 1987), males were seldom completely sterile. Because they maintain most of their endocrine competence, these salmonids also exhibit secondary sex characteristics, and as a consequence are susceptible to disease and exhibit no improvement in growth over diploids. Likewise, female triploid Atlantic salmon are commonly found to have a few normal oocytes within the ovarian matrix (Johnstone, 1993). Like CSM, inter-hybridization (or hybrid production) is a labor-intensive process that does not always result in sterility, as is clearly the case with the hybrid striped bass.
Gonadotropin-releasing hormone (GnRH) is a pituitary hormone that is required to maintain a normal reproductive cycle in vertebrates. Specifically, GnRH stimulates the synthesis and secretion of the gonadotropins: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Generally, the gonads are the primary target organs for LH and FSH. LH and FSH are integral to the reproductive system and inhibition of GnRH signaling and, therefore, disruption of the synthesis and secretion of LH and FSH is a potent method to induce infertility.
Disruption of the GnRH pathway has been accomplished in several species of fish by the introduction of a transgene that encodes antisense RNA that blocks endogenous GnRH expression [Uzbekova S, et al., “Transgenic rainbow trout expressed sGnRH-antisense RNA under the control of sGnRH promoter of Atlantic salmon.” J. Mol. Endocrinol. 2000; 25: 337-350; Hu W, et al. “Antisense for gonadotropin-releasing hormone reduces gonadotropin synthesis and gonadal development in transgenic common carp (Cyprinus carpio).” Aquaculture 2007; 271: 498-506.]. Some studies have shown however that low levels of GnRH expression persist in the transgenic fish resulting in a failure to completely induce sterility (Uzbekova S, et al., “Transgenic rainbow trout expressed sGnRH-antisense RNA under the control of sGnRH promoter of Atlantic salmon.” J. Mol. Endocrinol. 2000, 25: 337-350)
Another disadvantage of this strategy is that it is difficult to maintain a fertile population of fish for brood stock. Since the gene encoding the antisense RNA is integrated into the genome and continuously expressed, all of the fish will carry it, making it necessary to administer exogenous GnRH to individual fish by injection to maintain a fertile brood stock population.
Additionally, GnRH injection of brood stock is not practical in a large-scale commercial aquaculture operation.
Treatment with γ-aminobutyric acid (GABA) has also been proposed to disrupt the GnRH signaling pathway in fish [U.S. Pat. No. 7,194,978.]. Since GABA regulates GnRH neuron development in the embryo, treatment with exogenous GABA is able to disrupt the formation and normal migration pattern of the GnRH neurons [Fueshko S M, et al., “GABA inhibits migration of luteinizing hormone-releasing hormone neurons in embryonic olfactory explants.” J Neurosci 1998; 18: 2560-2569.]. Although this approach has been used successfully in the laboratory, it is not practical on a commercial scale due to the expense and labor required to treat large populations of fish. Also, the treatment affects other physiological and neurological functions in addition to gonad development in the fish.
As a result of the above constraints, the production of sterile fish, although considered highly beneficial to commercial aquaculture, has not yet been developed for mass use in the industry. Thus, it would be advantageous to develop a method and system to induce permanent sterility in fish grown in commercial operations that overcomes the problems of the previous unsuccessful methods used for sterilization.